The present invention relates to a method for both milling well casing and/or liner and subsequently drilling rock formation without the sequential removal of a milling assembly and replacement with a drilling assembly.
When an existing cased oil well becomes unproductive, the well may be sidetracked in order to develop multiple production zones or redirect exploration away from the unproductive region. Generally, sidetracking involves the creation of a window in the well casing by milling the steel casing in an area either near the bottom or within a serviceable portion of the well. The milling operation is then followed by the directional drilling of rock formation through the newly formed casing window. Sidetracking enables the development of a new borehole directionally oriented toward productive hydrocarbon sites without moving the rig, platform superstructure, or other above ground hole boring equipment, and also takes advantage of a common portion of the existing casing and cementing in the original borehole.
Conventionally, sidetracking to develop a new borehole has required at least two separate steps, the first step requiring the milling of a window in the original well casing and the second step requiring the drilling of formation through the newly formed window to create the new borehole.
The first milling step is performed by either directly milling an entire elongated section of pipe casing or by milling through a particular area within the side of the casing with a mill guided by a directionally oriented ramp, or a whipstock. U.S. Pat. No. 4,266,621 describes a milling tool for elongating a laterally directed opening window in a well casing. The disclosed system requires three trips into the well, beginning with the creation of an initial window in the borehole casing, the extension of the initial window with a particular cutting tool, and the elongation and further extension of the window by employing an assembly with multiple mills.
By integrating a whipstock into the milling operation and directionally orienting the milling operation to a more confined area of well casing, the number of trips required to effectively mill a window in a well casing have been decreased. A whipstock having an acutely angled ramp is first anchored inside a well and properly oriented to direct a drill string in the appropriate direction. A second trip is required to actually begin milling operations. Newer methods integrate the whipstock with the milling assembly to provide a combination whipstock and staged sidetrack mill. The milling assembly is connected at its leading tool to the top portion of the whipstock by a bolt which, upon application of sufficient pressure, may be sheared off to free the milling assembly. The cutting tool employed to mill through the metal casing of the borehole has conventionally incorporated cutters which comprise at least one material layer, such as preformed or crushed tungsten carbide bonded to a carrier, designed to only mill pipe casing. The mills used for milling casing are not suitable for extensive drilling of rock formation.
Once a sufficient window has been created, the milling assembly is removed and the drilling assembly is inserted into the borehole and directed to the newly formed window to drill earthen formation. Directional drilling is achieved by a number of conventional methods, such as steerable systems, which, when used, control borehole deviation without requiring the drilling assembly to be withdrawn during operation.
A typical system may use a bottom hole motor with a bent housing having one fixed diameter bit stabilizer below the housing and one stabilizer above the housing in combination with a measurement-while-drilling (MWD) system. Deviation is achieved by using the motor output shaft to rotate the drill bit while avoiding rotation in the drill string, thereby taking advantage of the alignment offset between the drill bit and motor generated by the bent housing. Angular variations of as high as 3 to 8xc2x0 per 100 feet (30 meters) are possible in such a system. Proper rotation of the drill string cancels angular deviations and can provide for an essentially straight drill path. Deviations, however, continue to occur at rates up to one degree per 30 meters as a result of variations in hole conditions, geological formations, and wear on the drill bit. Such variations can be corrected by steerable drilling assemblies.
Although drilling is often with a downhole motor operated at the end of a non-rotating drill string, one may also drill in a well borehole with a conventional rotating drill string.
The drilling of formation by the mill that cuts through the casing is limited in proximity to the creation of a xe2x80x9crat holexe2x80x9d near the existing borehole extending a distance of about five meters from the window through well casing. The milling assembly is fairly long and a rat hole is drilled into the formation to assure that the entire milling assembly passes through the casing and a complete window is made. A complete window is needed since the bits used for drilling rock formation are generally not considered suitable for milling casing. The rat hole is shorter than the bottom hole assembly used with the casing mill. Once the rat hole is complete, the milling cutter and bottom hole assembly is removed and followed by a third trip with a formation drilling assembly which then extends the borehole from the end of the rat hole to the next liner hanger point, the true end of the hole, or to an area proximate to the production zone being tapped.
Due to the high cost of oil well operations calculated both on a time and fixed cost basis, the current milling and drilling operations which require the insertion and removal of, at minimum, two separate tooling assemblies is inefficient and costly. Considerable time is lost round tripping tools in a well. A more cost effective approach to sidetracking would employ a method and incorporate the requisite devices which would both mill a window in the original well casing and subsequently drill formation through the newly created window in a single step.
It would be desirable to provide a method and device which enables the milling of pipe casing and subsequent drilling of formation without requiring multiple trips.
The present invention employs a dual-function cutting tool that is capable of milling pipe casing and/or liner and subsequently drilling formation. An exemplary cutter embedded in the cutting tool comprises at least a first material layer, such as cemented tungsten carbide, capable of milling pipe casing and/or liner and at least a second material layer, such as polycrystalline diamond, capable of drilling formation, the two layers being bonded together and to an insert body. The thickness and configurations of the material layers relative to each other and to the carrier vary and may include beveled and twin edge constructions which vary the cutting surface and improve the milling and drilling operation.
The cutting tool body is attached to a bottom hole assembly that connects to the drill string. The cutting tool may be optionally attachable to a whipstock to integrate the packing, anchoring, and orienting of a whipstock with the insertion of the milling and drilling assembly, thereby eliminating the need for a separate whipstock placement trip.
The milling and drilling process is conducted by shearing off the connection between the whipstock and cutting tool and directing the dual function milling and drilling assembly down the whipstock incline toward the well casing. After a window is milled through the casing, directional drilling can then proceed by any conventional method. The same cutting tool is used for both milling the casing and drilling the rock formation beyond the end of a traditional rat hole to the next liner hanger point or to the true end of the well.
Because the dual-function cutter eliminates the need to remove a milling assembly after creating a window in the pipe casing and subsequently send down a drilling assembly, the present invention provides a method which minimizes trips required to effectively sidetrack an existing borehole.